Beam amplifier



Jan. 9, 1951 D. E. WOOII DRIIDGE 2,537,388

BEAM AMPLIFIER Filed May 14, 1947 ,5 SCOPE 1 5; FIG. 4 f

' AMPLIFIED i 'v 0U TPUT X F IG. 9

L RA/A Z2 24 i l I N l E N TOR 0. E. WOOL DR/DGE A TTORNE V Patented Jan. 9, 1951 BEAM AMPLIFIER 7 Dean E.-Wooldridge, North Hollywood, Calif.-, assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 14, 1947, Serial No. 747,888

This invention relates to a process of inducing electrical conductivity in insulators and applications. thereof in electrical apparatus.

' In the field of nuclear physics there is a basic need for a device which yields a relatively large pulse of electrical current to correspond with .each' incidence on the device of a charged particle produced in the process of radioactive disfilled by gas-filled devices such as the well known Geiger-Mueller tube, which contains two or more electrodes across which an electrical voltage just insufficient to produce gas discharge is maintained. When an alpha or beta particle penetrates this gas, the ionization produced serves by a chain effect to cause local electrical breakdown of the gas. The pulse of electrical current flowing as aconsequence of this breakdown serves to operatea ccunting'device, which can be used to, determine the intensityof the radiation of charged particles from the; radioactivematerial'or'process under investigation.

An alternative application of this principle is the ionization counter, in which'true breakdown of the gas does not occur, but in which a pulse of current is produced whose magnitude corresponds to the number of ions produced by, and

determination of kind and intensity of radiation,

and more particularly, the radiation of charged particles. 7

A more specific object of the invention is to reduce substantially the size of apparatus em ployed in determining the kind and intensity of radiation of charged particles from radioactive materials.

Another object of this invention is to provide an external control for the flow of electrical current through a solid material across which an electrical field has been applied.

"The basic feature of manyelectronic devices of greatimportance to theart is the control of thefiow of electrical current between two electrodes in an evacuated or gas-filledspace by electrical means independent of the voltage applied between these electrodes. Among the many applications of this principle are the amplifica- 5' Claims. (01. 179---17l) tion of electrical signals in the well-known evac integration. Hitherto this need has been fuluated triode' and modifications thereof. All such apparatus hitherto devised for operation at normal temperatures have relied essentially on the control ofthe flow of electrons through a vacuum or a gas. Great advantages, especially by way of increased amplification accrue if such devices may 'be supplemented and some cases replaced by a' device in which the electron flow through a solid-material is similarly subjected to independent external control.

Another object of this invention is to provide an amplifier of-electrical signals whose amplifi cation for a given interelectrode spacing is greater than that hitherto obtainble. T

For the purpose of classifying substances in accordance with their electrical conductivities three designations are commonly employed. Electrical conductors connotes that class of substances which at room temperatures, that is; 15 C. to 25 C., have specific resistances of the order of lllfiohm-cm. Semiconductors include substances which=under the sameconditions ohm-cm. Insulators are substances which under the'same conditions have specific resistances of the order of 10 ohm-cmnand higher.

"The modern theory of the solidstate satisfac torily accounts; for the functioning of good elec-' trical insulators. This theory is presented in the early part of a paper by W. Shockley,- The Quantum Physics of Solids, beginning on page 645-; vol. XVIII (1939) of Bell System Technical Journal in which see especially pages 652 to 655.- Such materials owe their non-conducting properties 'to an equilibrium distribution of the electrons of the solid among the available energy levels in such a way that anappreciable energy difference exists between the highest filled level and the lowest unoccupied level. This energy gap, which may be several electron voltswide, is an effective barrier that prevents the transition of electrons into higher states under the influence of a field. Since there are no empty levels below the forbidden energy region for electrons to move into, no change in the over-all electron distribution can be produced by an applied field; hence no current can flow.

The condition of equilibrium in an insulator can be upset in several known ways. For exam ple, if the forbidden energy gap is sufficiently narrow or the temperature is sufiiciently high,- electrons occasionally can be thermally excited so as to occupy the unfilled band. Once arrived there, theyare free to move into higher energy 3 states under the influence of a field; in addition, the holes left in the normally filled band of levels make it possible for shifts to occur among this group of electrons when a field is applied.

pletely stopped in air. It heretofore :hasnotzbeen possible to make a direct measurement of the number of ions produced in a-solid; -but.-,the-.order of magnitude of that figure may be arrived at fairly simply. The argument is based onthe assertion that the interaction between an alpha pa ti l d an atom in agas cannotzloc r atly different from theinteraction between the alpha particle and-a similar atom in a solid. This ar c s strong experimental. upport from measurements on the stoppingd-power per 615. 111 of various elements, which .increases smooth y With increa i g a omic -weish a s ple empirical relationship fits -the observations, even though someelements are-solid and some are gaseous. Therefore, -it,--se,emsisafe .to conclude that the ratio sof v;.electrons knocked loose to atoms traversed willnot be far :difierent for. the carbon-atoms of diamond :thanaior the nitrogen of h t r willthe mcanene sy e p nded per electron be greatly different. Hence it has been inferred that an alpha particle of radium should raise about 1'0 electrons into the conduction'band on being stopped;inadiamondcrystal; i.-.e. should shift the electron from a condition where it cannot move in an {electric :field :to-a condition where it can.

When the alpha particle has transferred .the electron out of its normalenerg-y leveland into theconduction band, the .electron movestoward theaainode andzthehole in the-:f lledband noves toward the cathode if an eleictricfield is applied to the diamond. If suitable electrodes are pro- .videdion the crystal, vso that an electric field can be applied, these electrons and fholes -nove toyard opposite I electrodes; this ;gives rise to .:a .conductivity pulse in an appropriate measuring ciroilt- In. a similar Way, conductivity vis observedin diamo ry tal bo a ded b be -ra s for by electrons of moderate speedscwh-en. the.;mea sur-- ing circuit is made appropriate to these :observa-. ,tions.

Devices consistingof crystalline insulators having electrodes of gold, platinum 'or aluminum, deposited by evaporation from ihotwi-res of these materials have been successfully. used in producing bombardment induced conductivity. -.-Apparently a very intimate contact-between the diamond or other insulating body and the electrodes is desirable for the utilization of this .induced conductivity.

Inaccordance with this invention when certain crystalline bodies of insulating material, as .e. g, diamond are bombarded with leither alpha-particles, beta particles,- electronsof moderate speeds, they temporarily become electrical conductors. Advantage may be 'takenof, .for example, this phenomenon to detect the presence of streams ,of alpha particles, beta particles or .electrons of moderate speeds and to measure their intensity.

In certain embodiments of the invention the principle of bombardment induced electrical conductivity may be utilized by exposing a diamond crystal to an incident beam of electrically charged particles and simultaneously impressing on the diamond an electric field to produce a current throughthe diamond whenithe secondary electrons arereleased therein by the ionizing action of the incident particles. This current may be employed to actuate an oscilloscope to show the .nature and intensity of the incident particles or "it'may'be used to operate a counter to show the number-of particles incident upon the diamond withina given-period. If the insulator target in which electrical-conductivityis to be induced by bombardment be placed in the path of an electron beam which .is subjected to the control of weak electrical impulses to be amplified, the current resulting from the bombardment induced electrical conductivity may be very much larger than the wealrelectricalimpulses, thus enabling the device to serve'as an-electric amplifier.

A further objector-the invention is ,toincrea-se the ratio attainable between the number of ;;free charged particles released by bombardment and the number of primary (bombarding)electmns. A related object is to takeadvantageof; theincreased volume density-of a solid insulator type of bombardment induced conductivity device as compared with the alternative device using 'a gaseous medium, in theinterest of compactness and-economy of space.

Other objects of the inuention willbe-undem stood from the specification hereinafter follow ing, With-referenceto the accompanying drawings in which: 7

. Figs. ;1 and 21illustra-te alternative methods of applying the necessary-.-diffierence--of potentialto the surfaces of the insulators question, with relation also to the incidence of the bombarding particles;

Fig. 3 illustratesa systcm'oftheinventionfor indicating the presence of conductivity-in an i-n sulator which is affectedby-the bombardmentof charged particles;

Fig. 4 illustrates, toapproximatescale, an actual oscillographic record of the-inducementlof conductivity in a diamond by, means of -a- Fig, 3 type of circuit. 7

Fig. 5 illustrates a syste imilar 'to thatmf Fi 3 for indicating the presence of bombard ment induced conductivity in an insulator, here specifically an apparatus-for actually counting the incident charged particles, not restricted; to a particular type of source.oLbombardingpar! ticles;

. Fig. 6 illustrates an -.amplifier embodying the invention. 7

Fig. '7 illustrates a cross-section ture of Fig. 6 along the line Fi -.1; A

Fig. 8-shows a detail of the :quartz target of Fig. -6; and I Fig. 9 shows a modification of-the portionof the output circuit of Fig. 6 at the right :of line XX.

It should be understood, in what .fol-lows,,-tliat, with reference to anyparticular system for demonstrating or measuring the bombardment induced conductivitylof a diamondv or the like, the incident ray orbeam may :almostimpartially be made up of various-common types of charged particles. Applicable types-of charged particles include ordinary 'electrcnsas .typifiedby the cathode emanations in the .usual electronic devices, beta particles, which-are essentially high speed of he :rstr e electrons, and alpha particles which are positively charged particles. .Alpha and beta particles usually, and as contemplated by the present dis; closure, emanate from radio-active materials. -It should be understood also that circuits or systems for evidencing the f acts of bombardment induced conductivity do not differ in concept depending; on whether theultimate result is .a graphicalshowing of the conductivity, as on the oscillograph screen, or an audible response in a devicefor taining a quantitative measure of the incidence,

of the beam of charged particles; This is true although certain figures of the drawings are so differentiated in order to indicate such choice of means in the interest of special considerations.

Th foregoing generalization also-applies to the particular electrode systemsand Figs. 1 and illustrate two kinds of system that may be almost impartially used in any of the systems being-'de;

scribed, although a particular choice may be urged by particular practical considerations. Thesetwo systems differ in the nature of thecouplingof the electrodes to the solid dielectric substance on which they are superposed. In Fig. l the two electrodes are mounted on the same surface of the diamond in a side-by-side presentation, and

therefore so that the bombarding particles. need only affect thediamond more or less superficially,

" whereas in Fig. 2 the electrodes are mounted on opposed surfaces of the diamond so that the conduction current representsa phenomenon existing throughoutthe mass of the diamond,.thus implying that in the system of'Fig 2 the'bombarding particles and the particles released by the bombardment likewise affect the whole mass of the diamond .Referring to Fig. 1 more specifically two conducting metal film electrodes I and 2 are mounted on one. surface of the insulator 3... The gapA, separating the electrodes, is relatively small and various widths from .001. to '.008 inch have been successfully used- These electrodes may be prepared by the diamond surface roughly .in-half. by stretching a wire of appropriate diameter across and in close beindicated by device V may be applied across the diamond, that is, between the electrodes particle; the electrodes would not necessarily -,im-

pose a substantial barrier. Later .lnumbe'red figureswill show, more specificallyand in detail,

organizations including the elements. which .are

here shown to a large extent diagrammatically.

The angle of incidence is not critical.

Moderate electromotive forces applied between these electrodes by source '6 produce relatively high electric fieldsin the top surface layers of the diamond and the resultant induced conducplacement. Her the electrodes l and 2 are placed i about. A; inch in diameter (if circular. specimen is similar to that of Fig. 'l, as it might W811 be,.this could be very approximately either assvgass on'opposite surfaces of the diamond'3. A typical diamond specimen for this purpose mightbe If the from .the alternative of Fig. 1 in which the pulses pass in the region of the front surface and ina direction along the surface. J

In Fig. 3, illustrating a practical embodiment of a system operating according to the principles enuciated-with respect to Figs. 1 and 2, especially Fig. 2, similar elementsare, again, designated by like reference characters. The diamond .3- is coated with metallic electrodes I and 2 as in Fig. 2. The whole is mounted in an evacuated receptacle]. The charged particle source 8, first assumed as a source of'alpha particles, may con- Sist of a small piece ofsilver sheet 9 on which is deposited alayer of radium sulphate having'a given surface density of the radium component (in a typical instance, twelve micrograms of radium per square inch). The reference character ['0 indicates diagrammatically the support for the silver sheet. In the prior art there are adequate teachings of mountings similar to this and. other elements here disclosed in an evacuated container. Other facilities, likewise taught byth'e prior art could be used to advantage such as a magnetic control means to determine the particular direction of incidence of the particles on the diamond or even to adjust the position of the alpha particle source opposite the aperture II in a diaphragm-like element l2 for further determining and limiting the precise coactionof the beam of charged particles and the diamond.

"The same illustrat'on' is applicable to the use of a beta particle source. In this instance the element 9 could have the form of a piece of glass on which a minute quantity of artificially radio active strontium has been deposited. I

An adjusted potential, the value of Whichmay thereof, by primary source and potentiometer, together indicated by 3. The bombarded sur"- face of the diamond may be made either positive or negative, with relation to the opposite surface, by meansof the reversin switch ML Of course, in the specific instance of Fig. 3, the bombarding particles penetrate the exposed electrode before afiecting the diamond. This action does-not represent a significant departure from an alternative in which the diamond is directly bombarded. The detecting circu't comprises amplifierl5 and cathode ray oscilloscope !6, both shown diagrammatically to suggest the compara- 'tively impartial choice of specific means" to achieve these functions.

It is not a rigid requirement that the conta ner be evacuated. In practice, a rough vacuum is producedmerely to eliminate small induced conductivity pulses caused by ionization of the air produced by the charged particles in their transit to the diamond. These small efiects may alternatively, or in cooperation with the use of a vacuum, be largely eliminated by mounting the 7. zpartileisourcezasaclosetastrpracticableitoathevdia- :mon'd; lithisttherefore:requiringithatithe :diamon'd 633S0l1f08 8iaand diaphragm. all .betvery closely iinterspaced.

:Fig. :4; illustrates graphically,1-to approximate iscale,.rthe .operation' :of {an i'embodiment :of the invention in. a' circuit similar ito that' 20f Fig. :3 in -which.aIpharparticles emitted. by the radium fcomponent. of radium sulphate 'constitute :the tchargedtparticles. The-.drawingshowsfan actual sequence :of traces on an oscilloscope Ifield, the ordinates representing the current through the zdiamond,uas induced by the bombardment :by ithercharged particles, and the horizontal-axis :indicating. time. The oscilloscope pattern .-rep'- .:resented by said Fig. 4 was reproduced vironra photograph of one-twentieth :second exposure. --"A;:potential otoneor two hundredivoltswasapgplied between the electro des, thebombard'ed electrodebeing connectedto the negative sidezofthe =battery. .Each one of the pronouncedflvertical cusps (displacement .of the cathode ray) represents an induced oonductivitymulse produced iini the diamond by bombardment with a :single zalpha particleiandlasting aneexceedingly small =fraction:of a-as'econd. These alpha particle-emamations occur at random along thes'horizontal \trace. in conformity with the well-knowniact [that alpha particles are emitted; randomly in dain'recfrom radium. These zpulseseare shown to Tvaryiin height. A calibration of the particular detector circuit used revealed that the. maximum gpulseiheight corresponded to aJchargeof'atileast -.5 -.electrons. ltwas determined, at the. same -time, that a reversal of the relative. polarity .of the'diamondielectrodes caused-a reversal oithe .Fig. ri pattern, thedisplacements then occurring ..in.a' downwardly direction.

;.It is .noted that these displacements. aresupergposed ona background displacement b which; appears-zas a zone with irregular boundaries. in Fig. .4. Thisbackgroun'd effect is obtained. with the potential aremovedsirom the diamond electrodes, the displacements makingit. .up- -corresponding rtothe-noise'which is characteristic of the ampli- :fier. It is evident from Fig. .41that.theicircuitiof Fig. 3.--maybe used in thequantitativemeasure- .ments of the bombardment induced conductivity of the diamond, both. as to the intensity of-any igiven pulse and as to the number of pulses. By. increasing the abscissa range representing a small interval of time. and'by sufficiently.magni-- tying the pulse amplitudes, in accordance with conventional osc.lloscope practices, .it is quite practicable to-count and appraise 'the .efiect .of

the incidence .of single bombarding; particles- .An applied. fieldof 2,000 volts per-centimeter, or .evenless, is sufiicient to-developrthese. current pulses. For example, such: pulses have been obtained with only five volts appliedto the electrodes of the Fig. 1 type, as .embod'edcin the Fig. 3 circuits, and with'an electrode saparation 0f .001 inch. Similarly, using the Fig. 2 type-oi electrode placement, these current pulses have been developed with lessthan 1.00 volts 'between .theelectrodes where the diamondithicknesswas about .020 inch.

Observation of these induced conductivity .pulses,-in which amplifier had an extremely high speed characteristic, showed that the rise .time of the pulse observed on the oscilloscope was about 0.15 microsecond. Since thisfi-gure represents a=limitationin the amplifiepitsslfgit .is reasonable to .conclude from this observation that the true .rise of the :pulse is less than 0.15

lclesestsapproximation .to this invention; although iuszng: quite diffierenttypes of crystal and; not" the same variety of types.- of charged particles,.:re- .quired -a temperature: corresponding .tothat :of Iliquid; air or :thelike, 1and,:no:.practical*resultfwas .achievedrat. room-temperature.

It.has-fbeenspointedout that the circuit :ofl fig.

.3 lmayzhexused with beta-particle bombardment,

instead of with. alpha particle bombardment. .With- "beta particle bombardment observations isimilar'itouthosei illustrated. by Fig. i 4: havev been zmade and which. are :similar. in kind, :althoug'h .notxin magnitude', :tothose:.ofrsaid-Fig; 4. .There Vase 3M1: same: dependence of these pulses on'=the polarity ofitheaappliedipotential. Onthesavera'ge theilpulses; .were esmaller than :those' "of. the: :alpha particlesibyca-ziactor.:dii 4:01:15, Whi'ChliSitGi be :ex- :pected:since-thev alpha: particles :had four or fi've times the maximum: energy .of the beta particles r.used..

Fig. 5 emphasizes the embodiment ofi-the-iinventioniras'taccounter. Similar.elementsiareisiihillarlycdesignated; as in Eig; .3, ithe essential .dif- .ferenceheing ithe "use: of the counting device .20 linrplace ofiithe oscilloscope lifiof Fig. .3, it-being rrecognizedthatfithe :prior art provides impulse (pulse) counters of a large variety'and scopeand ihaving greater f-acility than the oscilloscope of :Fig. :3, a-ArpdtentialofQOO- 0r SOOvoIt Jmay :he :applied :"acrossitlre diamond' crystal. .I'he struoituretoixthetcrystal and its electrodes "maybe madertofoncupy aispaceieach of whose dimensions iszilessz :thanmne-rourth :ofxan inch. This strucrturezis :exposed' to. the desired source or radiation, the zparticlesifromiwhich are to be counted. .The: :cun'ent :pulses', .which new through the 1 diamond" each ltime'a :charged: particle penetrates iit;..=are iamplified. as YSIIOWII and th :signals "thus s mro'duced tare. counted'by the. counting device .20.,

:5 iwh-ichzmayhe Iad iusted so thatonly pulsesi'ofla igiven' :amplitude ;or; greater are .selected "for counting.

"Fig. :6; illustrates ."an amplifier iembo'diment of the invention. whereby .a wave to be. amplified c0 originating in circuit i2l :may be translated "in .amplifieclzfornrxtothe. output circuit 2:2 of'tthe bombarded. targetifiz-and thencemay be utilized lin oscilloscope-om counter .ZLsimila-rly as the imodifioationsrnfFigs. -3 and 5., or, as taught by 35 113118 alternative. output .circuit of .Fig. .9, may be .utilized for "any :other purpose. normally served .byiansamplified wave; \Of course; in :the Fig; .6 .circuitian amplifier may: be inserted "between the target :Z3 Ja-nd the responsive: means 24' :similarly -7 rasrin. Figs. -.3 and: 5*as irequired to condition; said iresponsivemeans Pi l -tor most'effectiveoperation. Morespecifica-lly the amplifier comprises-an evacuated container 25 bounded by conductive plates'l32 and :33, the'cross sectionior the tube beingrshownxinlllig. 7. A critical difierencecof relation-to the-:prior artrinr which probably the 9 potential between said plates is determined by source and potentiometer 21; which causes the upper plate 32 in thefigureto be negative with respect to the lower plate 33. Under these conditions electrons emitted by electron gun 28 are repelled from the upper plate so as to have a parabolic trajectory. enabling the electrons eventually to be incident on the target 23. a The wave from the source 2! affects the static electrical condition of the plates to the extent of varying the extent of the passage of the electrons to the opening 29 from which they are incident on said target 23. As has been. before explained,..the'

small asmechanical consideration'will reasonably permit it efiectively serves as a device of high re- I solving power to indicate variations of radiation strengths in-space as the device is moved to ex- I plore the space point by point. Counters according to the invention have been successfully used in which the diamond area exposed to alpha particles was only 0.001 inch by 0.2 inch The 0.001 inch width afiords exceedingly high spatialresolving power.

The high density (that is, his. absorbing power) of the diamond counter gives it an 'ad-.

vantage over the Geiger-Mueller counter in very fact of incidenceof a beam of charged.

particles, such as the electron beamin the "pres ent instance, on the target 23 connotes an am-. plified response in the output'of said target. By

the variable difierence of potential between plates" 32 and 33 as superposed by. source Elf-on the steady diiierence of potential'from source 21, this amplified response ismade proportional to'the waves to be amplified from source Zil The electron gun 28 is disclosed diagrammatically only since its specific details, likewise diagrammatically indicated in the figure, may conform with the conventional practice which permits a considerable variety of choice 'of specific means. f

In the organization of Fig. 5, as used by applicant, there were certain significant variants,

as to the bombarded insulator or target, as com-' of the opposed edges of the electrodes and as to their metallic constitution. The electredeplacement of thisFig. 'B isj believedtepromme the bombardment induced fcOnductiVity ofthe material and its sensitivity to change. f The counter of this invention 'can ,be used wherever the conventional Geiger-Mueller gas tube counter is used in nuclear physics studies where radioactive radiations are to be measured.

These solid insulator counters have been worked at as low as one or two volts applied across the electrodes, where the Fig. 1 type of electrode placement is used. This low voltage feature immediately suggests the possibility of the use of 3 these counters in rockets, weather balloons, and

the like where low battery weight, size and cost are important. The absence of electrical breakdown at high altitude is an advantageto be derived from the low voltage of operation or a counter embodying the present invention.

Because of its small size, the diamond counter can be inserted in small cavities to measure radioactive radiation therein. 'For instance, it isquite feasible to insert the diamond counter into the body cavities of an animal or a human being for radiation measurements and biological studies. Inherent in its small size is the further advantage, that the diamond will give a low background count due to stray radiation.

Because the diamond insulator counter gives a sufficiently great response even when made as These electrodes may be, as they measurements of very penetrating radiations,

which on passing through a gas tube may not los'e enough energy to actuate'it. This advan tage of the diamond is particularly important in measurements of'feeble sources of' penetrating radiation. r The diamond insulator counter has a high counting speed resulting from the small time required' to collect electrons freed by the alpha particles. A testhas shown that thiscollection time is at most 0.15 microsecond. There is reason: to think that' it may be assmall as 0.01 micro-1 second or even 0.001-.microsecond. This is'mu'ch energy of the alpha particle.

smaller tha'ntthecorresponding time for the Geiger-Muellertype of counter. This is particu-wlarly important in measurements of strong radio-.11,

active sources because the diamond counts more accurately, Geiger-Mueller counter and the chance of a single count resulting from several nearly. simultaneousalpha particles is .much smaller -,than;in the- Geiger-Mueller counter; 1

Since the diamond counter does" not need :to be. encased in a containerjthere are not necessarily. any absorption losses 'due to container ,walls.'. This is an-important advantage particularly in the measurement I .of radiations tof v short rangei Forinstance, the: diamond counter-can be immersed in a dielectric. liquid to .make radiation measurements of. radioactive substance in a solution. Animportantcharacteristic of the dia-emond counter is its intrinsic sensitivity... It has. been observed thatiin it one electron may be released for as'littl'e .as eachten electron: voltstoI In air the alpha. particle expends about 35 electronvolts of energy for each electron released by ionization. Thus the intrinsic sensitivitytofthe diamond counter may be several timesias great as that of the.

ionization counter.

As a further aid to the practice of the invention, especially as relating to a choice of the solid insulator that might be used, within the spirit of the invention, alternatively .to diamond, the following considerations are offered. Theoretical considerations would suggest the desirability of such an insulator beingof the single crystal type, having a specific resistance greater than perhaps 10 ohm-cm. and with a high degree of chemical purity and freedom from inelastic strain or other crystal defects. These considerations suggestthe possible use for this purpose of some or thereof the following insulators: Zinc sulfide. the alkali halides (particularly potassium chloride) ,tmagnesium oxide, calcium fluoride, quartz, sodium nitrate, topaz, silver chloride, orthoclase, beryl, calcitef apatite, selenite', tourmaline and emerald of which applicant has worked with zinc sulfide, magnesium oxide, calcium fluoride, and quartz It will be apparent that the invention provides a'useful tool for measuring a wide variety oilin at the higher speeds, than the; 

